Proportional pressure control valves (such as disclosed in U.S. Pat. No. 4,316,599) are known which among other things form control valves for oil hydraulic systems and deliver an essentially constant output pressure for a variable input pressure. The output pressure to be controlled is dictated by the current signal delivered by the corresponding trigger electronics and acting on the actuating magnet as the magnet system. The actuating magnet can be made as a pressure-tight oil bath magnet and has a long service life.
Proportional pressure control valves such as these can be directly controlled piston sliding valves in a three-way design, i.e., with output side pressure safeguarding. They are used among other things in oil hydraulic systems to control clutches in shift transmissions for controlled influencing of pressure build-up and pressure drop, for remote pressure setting, and for controlling pressure variations and for pilot control of hydraulic valves and logic elements.
These conventional proportional pressure control valves have poor stability especially for thin-liquid fluid media, i.e., they begin to “oscillate”. This problem is especially harmful when the known valves are designed to perform special functions, for example, in motor vehicle power steering systems, hydraulic drive units, and other safety engineering-relevant domains. It has generally been found that when pressure control valves are used, susceptibility to fault variables lies in the natural frequency region of the valve. The instabilities occurring can lead to failure of the valve and the pertinent parts of a hydraulic system.
In the prior art, multiaxle drive units for elevating work platforms are known which generally have a rear drive and optional all-wheel drive. To ensure safe operation with driving of only one axle and a free-running of the other axle and execution of braking processes with sudden stopping, in the known solution a plurality of valve components is necessary, such as two shock valves, two counter balance valve combinations. The valve combinations can include one pressure control valve and one 2/2-way valve each, and a 2/2-way valve as the recirculation valve. This functional structure is reliable in operation, but due to the plurality of valves its failure in operation must be expected. This failure shortens maintenance intervals. The known solution also requires a large amount of installation space and is expensive to manufacture due to the diversity of parts.